In order to improve the performance of the dynamic shaft seal having a sealing edge engaging a rotating shaft, as described above, the sealing edge has been provided a hydrodynamic assist. This hydrodynamic assist was initially provided by helically disposed ribs or grooves which, together with the rotating shaft, create a hydraulic pressure affect on the oil to force the oil back under the sealing edge. However, because of the helical disposition of the ribs or grooves, the hydrodynamic assist was effective for only one direction of rotation of the shaft.
Consequently, shaft seals have been developed to provide the hydrodynamic assist on the air or dry side of the sealing edge to pump oil which leaks past the sealing edge back under the sealing edge to the oil or wet side. Projections of various configurations have been provided on the frustoconical surface adjacent the sealing edge on the air or dry side to engage the shaft and pump oil back under the sealing edge to the wet or oil side. Examples of such pumping projections are shown in the U.S. Pat. No. 3,656,227 granted Apr. 18, 1972 to Louis H. Weinand, U.S. Pat. No. 3,586,342 granted June 22, 1971 to Thomas E. Staab, U.S. Pat. No. 3,873,104 granted Mar. 25, 1975 to Dean R. Bainard, U.S. Pat. No. 3,868,105 granted Feb. 25, 1975 to G. A. Bentley, and U.S. Pat. No. 3,895,814 granted July 22, 1975 to Bernard F. Kupfert et al. The pumping projections shown in these patents include one singular pumping surface for engaging the shaft.
Under some operating conditions, the shaft may be misaligned or run eccentrically. When the shaft runs eccentrically, the axis of the shaft is offset from the axis of the seal whereby the shaft runs in tighter or firmer contact with one peripheral portion of the seal and in looser contact with the portion of the seal 180 degrees about the seal from the first portion. On the light contact side, the pumping projection might not be in contact with the shaft at all with only the sealing edge being in contact with the shaft or, if in contact, may not yield an effective hydrodynamic pattern. On the high contact side only the rear heel of the pumping projection may be contacting the shaft with the adjacent sealing edge out of contact with the shaft, thus resulting in leaking.